Exactly. I'm quite surprised nobody mentioned this. Who preserves a kubel more than 10 minutes from game start? Don't tell me Helping Hans or such. I don't think we are discussing at that level. It's not comparable to any infantry unit including maxim, which can be usefull all the game.
That is why it's not OP. Not by far.
I legit got a Kubel to Vet-4 pre-buff once.... once...